LeetCode 230. Kth Smallest Element in a BST



Given the root of a binary search tree, and an integer k, return the kth (1-indexedsmallest element in the tree.

Example 1:

Input: root = [3,1,4,null,2], k = 1
Output: 1

Example 2:

Input: root = [5,3,6,2,4,null,null,1], k = 3
Output: 3


  • The number of nodes in the tree is n.
  • 1 <= k <= n <= 104
  • 0 <= Node.val <= 104

Follow up: If the BST is modified often (i.e., we can do insert and delete operations) and you need to find the kth smallest frequently, how would you optimize?


In-order traverse to get node values in ascending order. Then get the k – 1 th value.

Python Solution

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def kthSmallest(self, root: TreeNode, k: int) -> int:        
        values = []
        self.inorder_traverse(root, values)
        return values[k - 1]
    def inorder_traverse(self, root, results):
        if not root:
        self.inorder_traverse(root.left, results)
        self.inorder_traverse(root.right, results)
  • Time complexity: O(N).
  • Space complexity: O(N).

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